Atomic Structure Chapter 4

Studying Atoms 4.1

Ancient Greek Models of Atoms Democritus Believed that all matter was made of small particles that could not be divided Called the particles atoms (atomos means uncut) Aristotle Did not think there was a limit to number of times matter could be cut

Aristotle’s View

John Dalton Studying gases Concluded gas was individual particles because of the way they exerted pressure Measured the masses of elements, discovered all compounds have fixed compositions Modeled atoms as solid spheres

Dalton’s Theory All matter is made up of individual particles called atoms, which can’t be divided. All elements made of atoms Atoms of same element have same mass, atoms of different elements have different masses Compounds have more than one type of element Atoms of different elements always combine the same way in compound

JJ Thomson Used electric current to study atoms Thomson thought atoms had charged particles in them He set up a tube with positive plate and a negative plate, the beam of light was either repelled or bent towards the plates First evidence that atoms are made of even smaller particles

Thomson

Thomson’s Model Had evenly scattered positively and negative charges in the atom Called this the “plum pudding model” Think chocolate chip ice cream

Ernest Rutherford His assistant aimed a beam of particles through a slit so that they would hit gold foil, he though that many would be so small they would go in a straight line and hit a screen that would light up Many particles were deflected though

Rutherford

Rutherford’s model Concluded that the positive charge of the atom is not evenly distributed Concentrated in a very small, central area- called the nucleus Nucleus = a dense, positively charged mass at center of atom Negative charges (e-) traveled randomly outside the nucleus

The structure of the atom 4.2

Parts of the atom Proton Neutron Electron

Sub-Atomic Particles Protons = Positively (1+) charged particle in the nucleus Neutrons = neutral particle in nucleus Electron = negatively (1-) charge particle in nucleus Protons, electrons and neutrons can be distinguished by mass, charge and location in atom

Atomic number Atoms of an element always have the same number of p+ Different elements have different atomic numbers Atomic Number = the # of protons in an atom Can find on the periodic table Equals the number of electrons (atoms have no charge so + = -)

Mass number Mass Number = sum of protons and neutrons Number of Neutrons = Mass number – Atomic Number

Isotopes Atoms of the same element but have different numbers of and neutrons Atomic mass is different because number of neutrons is different

Property Ordinary Water Heavy Water Melting Point 0.00 ⁰C 3.81 ⁰C Boiling Point 100 ⁰C 101.42 ⁰C Density .99701 g/cm3 1.1044 g/cm3

Modern Atomic Theory 4.3

Bohr’s Model Atom had nucleus with p+ and N (agreed with Rutherford) Electrons moved at constant speeds in fixed orbits around the nucleus (like planets and sun) If atoms gain or lose energy the energy of the electrons changed Electrons had energy levels Electrons could move up electrons levels or down electrons levels (like stairs)

Bohr’s Model Energy e- gain and lose can be measured We can see some of the energy released by e- as visible light Fireworks: Explosions cause e- to move to different energy levels, when e- move back down energy levels they release light No two elements have the same energy levels so we get different colors of light

Bohr model

Electron cloud model Atoms have energy levels, but do not move like planets Electron cloud model = most likely location of e- in an atom, denser the locations have higher probability of find an electron Like a fan – when still you can count fan blades, when it is on high they look like a blur, you know they are there but can’t not exactly sure where they are

Electron cloud model

Atomic orbitals Orbital = region of space around the nucleus where e- is likely to be Example: Map of school, you place a dot every 10 minutes of where you would be, after a week you would have your orbital, a place where you would likely to be found Electron cloud = good approximation of how e- behave in orbitals Lowest energy level has 1 orbital, higher energy levels have more orbitals Each orbital can have 2 e- at most

Energy levels, orbitals and E- # of Orbitals Max. # of E- 1 2 4 8 3 9 18 16 32

Electron configurations E- configuration = arrangement of e- in orbitals of an atom Most stable e- configuration is the one in which e- are in lowest energy level possible Ground state = when all electrons in atom have lowest possible energy levels Excited state = when e- move to higher energy levles